Prior to the discovery of polymers, packaging materials included paper, glass, and metal; materials which were often ineffective and costly in the storage and transfer of perishable foods. The implementation of polymers into packaging materials tremendously improved the preservation of perishable foods. In addition, with improved biological control and faster and better means of transportation, the availability of perishable food products in general is no longer tied to a geographical area, or to seasonal factors.
Generally, the materials used to contain, protect, preserve, and inform the consumer play a critical role in the sale of perishable products. Often, desirable polymer properties in packaging films are those of barrier to oxygen, carbon dioxide, and moisture. However, with perishable food products such as fresh fruits, vegetables, and cut flowers, which contain living cells that respire, transpire, and conduct other metabolic processes long after their harvest, a desirable polymer film provides high, adjustable transmission rates of oxygen, carbon dioxide, and moisture. Respiration and transpiration are the main processes that affect the atmospheric composition inside a package. Ideally, moisture should be retained just to the extent to avoid condensation when temperature changes occur (for example, when the package is placed inside a refrigerator). Respiration and transpiration are regulated by temperature. As a general rule, for every 10xc2x0 C. increase in temperature, the respiration and transpiration rates are doubled or tripled. Also, an exchange of carbon dioxide and oxygen with the environment has to take place if the produce continues the respiration process.
One of the difficulties with post-harvest preservation of produce is that each product has specific needs. Some respire faster than others and the same produce will respire at different rates, depending on the part of the plant, elapsed-time after harvest, growing season, growing conditions, area of production, type of cultivar, etc. Those that respire faster require more refrigeration than those that have slower metabolic rates. For example, asparagus respires ten (10) times faster than a tomato and an apple maintained at 20xc2x0 C. ripens three (3) times faster than one kept at 10xc2x0 C. Also, the faster a product respires, the greater the rate of sugar consumption, and the greater the amount of heat generated. Thus, refrigeration is essential for preservation because water and sugar loss critically affect the quality of any produce. Therefore, the storage life of perishable products varies inversely with respiration.
After refrigeration, the most important resource for the extension of produce shelf-life is the use of modified atmosphere packaging (MAP). Upon harvest and packaging for storage, perishable goods must be immediately conditioned for longer preservation. Active atmospheric control is done by packing the product in bags that are filled with an xe2x80x9coptimum compositionxe2x80x9d of oxygen and carbon dioxide, and balanced with nitrogen. The optimum composition for each product is the atmospheric composition that is adjusted to the respiration rates of the produce at a given temperature (i.e. neither more nor less than what the product needs). For this initial composition to be maintained for a long time, the gas permeation rates of the bag should be matched to the product needs. In this aspect, both the material and the physical design of the packaging play a role. MAP technology is extremely dependent on the flexibility of film permeation rates when applied to different produce and combinations of produce inside a single package. Unless the barrier provided by the packaging matches permeation rates with content respiration rates, an imbalance builds up, accelerating aging and facilitating bacterial or fungal growth.
There are packaging films for perishable foods currently available that exhibit variable gas permeation based on temperature. U.S. Pat. No. 5,254,354 describes packaging films composed of side chain crystallizable (SCC) acrylate polymers that exhibit temperature-sensitive gas permeability. Polyethylenes were included among a list of various polymers described as unsuitable for use as thermally responsive packaging material, based upon the resulting film""s poor gas permeability response to temperature.
U.S. Pat. No. 5,665,822 discloses elastomers containing side chain crystalline blocks that exhibit increased permeability to oxygen and carbon dioxide when their crystalline side chains reach their melting point. These elastomers are prepared by polymerizing acrylates to form polymer blocks having crystalline side chains of either polymethylene moieties or perfluorinated ethylene moieties and linking these polymer blocks with other polymer blocks consisting of polyalkanes.
Additional films exhibiting gas permeability characteristics are disclosed in U.S. Pat. No. 5,685,128 (the ""128 patent) and U.S. Pat. No. 5,958,319 (the ""319 patent). The ""128 patent discloses packaging films composed of a xe2x80x9csubstantially linearxe2x80x9d ethylene polymer and polymer blends of substantially linear ethylene polymer with homogenously branched ethylene polymer to provide increased oxygen permeability. The ""319 patent discloses methods for producing polyethylene films using metallocene catalyst systems. By manipulating various processing parameters, these films exhibit improved gas transmission rates at a given polyethylene resin density.
Current materials used to make packaging films for fresh produce are often composed of polyolefin polymers, such as polyethylenes. These perishable food packaging films exhibit only gradual changes in permeability as a function of temperature. As simple hydrocarbon polymers, polyolefin films are relatively simple to synthesize and process; however, it has been difficult to establish successful permeation control in polymers of the polyolefin family. Therefore, there remains an unfulfilled need for polyolefin films that exhibit sharp permeability transitions based on variation in temperature.
The present invention concerns polymer materials which exhibit temperature-sensitive permeability. The polymer materials of the subject invention are particularly useful for packaging goods or articles that require gas exchange for optimum shelf life, e.g., fresh vegetables, fruit, and cut flowers.
The subject invention is at least partly based on the surprising discovery that polyolefins containing a high concentration of short branches exhibit temperature sensitive permeability properties that make them useful for a variety of applications where control of permeability is desirable. In a preferred embodiment, the polymer materials of the subject invention contain a high concentration of branches of about ten carbon atoms in length or less. Preferably, the polymer materials of the subject invention contain about 50% to about 100% xcex1-olefin by weight. Olefins utilized in the subject invention are preferably between about four and about twelve carbon atoms per molecule, thereby producing branches of between about two and about ten carbon atoms in length.
Appropriate xcex1-olefins include, for example, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, and 1-dodecene. Preferably, the xcex1-olefins are selected from the group consisting of 1-butene, 1-hexene, 1-octene, 1-decene, and 1-dodecene. More preferably, the polymer materials are cc-olefin/ethene copolymers selected from the group consisting of ethene-1-butene, ethene-1-hexene, ethene-1-octene, ethene-1-decene, and ethene-1-dodecene.
The subject invention also concerns polyolefin films particularly useful for packaging perishable foods that require gas exchange. The polymer films of the subject invention have been shown to exhibit gas and water vapor permeabilities that are very sensitive to changes in ambient temperature. Therefore, these materials could be considered so-called xe2x80x9csmartxe2x80x9d materials that adjust permeation properties according to environmental changes.
The polymer materials of the subject invention can be formulated into various containers, such as bags, particularly suited for packaging fresh vegetables, fruits, or cut flowers. The polymer materials of this invention could be useful in some post harvest techniques for preservation of produce. In Modified Atmosphere Packaging (MAP) for example, the produce is placed inside a packaging article in the presence of atmospheric gases, such as oxygen, carbon dioxide and nitrogen, in concentrations that are known to extend the shelf life of the packaged product.
The polymer materials of the subject invention can be used to extend the preservation of fresh produce, reducing losses by spoilage during handling and storage, and extending the period of marketability. Improvement of the quality of the produce is also expected, since better preservation means ultimately that more nutrients and water contents, higher crispness, better freshness, color and flavor could be maintained for longer periods of time.
The subject invention also pertains to a package comprising a polymer material of the subject invention and an article, wherein the polymer material separates the article from the surrounding atmosphere. The subject invention also concerns methods for packaging articles by placing the article within a container composed of a polymer material of the subject invention. The polymer materials of the subject invention are particularly useful for packaging respiring articles that would benefit from gas exchange.
The subject invention also pertains to clothing, drug delivery devices, and ocular devices formulated from the polymer materials of the subject invention.